1. Field of the Invention
The present invention relates to an optical recording medium incorporating a recording portion, which is formed on either main surface of a supporting body and on which a signal is recorded, and a light permeable layer formed on the recording portion and arranged to apply light from a position adjacent to the light permeable layer to record and/or reproduce an information signal.
2. Description of the Related Art
An optical recording medium has been suggested which is capable of continuing a recording operation or a reproducing operation for four hours on one side thereof by employing an NTSC (National Television System Committee) method. The foregoing optical recording medium has a function to serve as a new recording medium which is substituted for the video cassette for use in a present VTR (Video Tape Recorder).
On the other hand, the optical recording medium has been required to have the shape and size which are similar to those of the CD (Compact Disc) so as to be a product which is free from uncomfortability for a user who has been accustomed to the easiness and ease-of-use of the CD. The optical recording medium must be formed into the disc-shape recording medium similar to the CD to be capable of using the random access and the quick access which are the greatest characteristics of the disc shape medium. Thus, a small and easy recording medium can be formed. Moreover, the optical recording medium is required to be a product which is capable of instantaneously performing recording and reproducing and which has a multiplicity of functions including a trick play and a quick editing characteristic.
The optical recording medium must have a variety of performance and characteristics so as to be used as a next-generation recording medium as described above. For example, a storage capacity of 8 GB or greater is required.
The conventional optical recording medium, however, has a poor storage capacity which is smaller than 8 GB. As a conventional optical recording medium, the DVD (Digital Versatile Disc) has been suggested. The DVD has a recording wavelength .lambda. of 0.65 .mu.m and an NA (Numerical Aperture) of 0.6. The DVD has a recording capacity of 4.7 GB.
To make the signal formats including ECC (Error Collection Code) to be similar to those of the DVD and to enlarge the storage capacity to 8 GB or greater, the optical recording medium must satisfy the following relational expression: EQU 4.7.times.(0.65/0.6.times.NA/.lambda.).sup.2.gtoreq.8
By solving the foregoing relational expression, NA/.lambda..gtoreq.1.20. To enlarge the recording capacity of the optical recording medium to be 8 GB or greater, enlargement of the numerical aperture NA and shortening of the recording wavelength .lambda. of the optical recording medium are required.
When, for example, the numerical aperture NA of the optical recording medium is enlarged, an allowance of an angle (a tilt angle) which is deviation of the surface of the disc from the optical axis of an optical pickup is undesirably reduced. An influence of the foregoing tilt angle is easily exerted on the aberration of the optical recording medium produced owning to the thickness of the disc shape optical recording medium. To record/reproduce a signal stably, the thickness of a light permeable layer, through which irradiation light passes, must, therefore, be reduced. Moreover, irregularity in the thickness of the light permeable layer of the optical recording medium must be not larger than a predetermined value because of the foregoing reasons.
When the thickness of the light permeable layer of the optical recording medium is reduced as described above, an advantage can be realized. On the other hand, an adverse and critical influence of a flaw formed on the surface of the disc or dust is exerted. Therefore, there arises a problem in that a signal cannot be easily recorded/reproduced. When recording and/or reproducing of the optical recording medium having a raised recording density is performed by using an optical system having a large numerical aperture NA, the working distance which is the distance from an objective lens and the surface of the disc must be shorter than that of the conventional optical recording medium. At this time, the possibility of conflict between the surface of the disc and the objective lens is raised. Thus, a flaw is easily developed. At this time, charging of the disc, which is the optical recording medium, causes the amount of dust or the like to undesirably be enlarged. Therefore, the optical recording medium has a problem in that recording and/or reproducing errors increase owning to the flaw and the dust.